The invention relates generally to thermoplastic packaging, and more specifically to heat resistant nylon multi-layer packaging film for food products.
It is common practice to package articles such as food products in thermoplastic films or laminates to protect the product to be packaged from abuse (i.e., oxygen or moisture) or exterior contamination, and to provide a convenient and durable package for transportation and ultimate sale to the end user.
The equipment used to package said food products typically use two multi-layer films to protect the product. One film is heated and formed into a die cavity where the product is loaded into the cavity. The second film is generally mechanically fed through the machine and allowed to drape or hang over the filled cavity and a heated metal platen pushes the second film (also known as the non-forming film) over the product and into the remaining space left in the cavity and the two films are sealed together with temperature, pressure and time.
Two major types of nylon used in multi-layer packaging film of this type are Nylon 6 and Nylon 6/6,6. It is well known in the prior art to use one type or the other in multi-layer films. Nylon 6/6,6 is a copolymer nylon that has a lower melting point (196.degree. C.) than Nylon 6, flows easily when heated and bonds to other polymers well for a polyamide. Its flow properties make it an excellent structural nylon for forming films that are used to form into the die cavities of packaging equipment. However, when film made with Nylon 6/6,6 is used as a non-forming film, a temperature related problem occurs. The problem manifests itself when the Nylon 6/6,6 non-forming film is exposed to the high temperatures required to produce hermetically sealed packages. Coextruded, multilayer films made with Nylon 6/6,6 will stick to the seal platen or in the worst case, physically melt and create holes in the film when exposed to the temperatures necessary to create acceptable packages.
Nylon 6 has a higher melting point (220.degree. C.) than Nylon 6/6,6 and as a result has more resistance to sticking or melting than Nylon 6/6,6 and is the preferred resin for most non-forming applications. However, it is known that using Nylon 6, in very demanding applications, such as sealing through product contamination or making packages at very high line speeds, can lead to the same sticking and melting issues observed with multi-layer films made with Nylon 6/6,6.
Often in food packaging applications a "fatty" residue is left on the packaging film when positioning a food product to be packaged. Knowledge of the heat seal process is vital to understanding the importance of heat resistance in a packaging film. The heat seal process is as follows:
1) The heat seal layers contact each other in the packaging operation when heat and pressure are applied; PA1 2) The sealant materials melt with time and temperature; PA1 3) The opposing heat seal layers are transformed into a "wetted" interface with time and pressure; PA1 4) Once the polymer molecules are "wetted," they begin to diffuse and entangle across the interface, forming an "interphase," a region of melted entangled polymer chains; and PA1 5) Heat is removed and the polymer at the "interphase" begins to build viscosity as it cools, eventually crystallizing into a solid heat seal.
The above "fatty" residue interferes with the wetting of the polymer seal layers and subsequent polymer diffusion that occurs during the heat seal process. The more heat one can apply to the packaging film, the lower the viscosity of the melted polymer which allows more diffusion and entanglement of the polymer molecules at the "wetted" interface. The more entanglement and diffusion that occurs, the greater the chance the sealant layers will not be affected by the contamination residue.
A material such as Nylon 6,6, which has an even higher melting point (255.degree. C.) than that of Nylon 6 and thus could potentially be used in high temperature heat seal applications, would be desirable.
However, it is well known that using Nylon 6,6 in a multi-layer structure creates temperature induced problems for the layers near such nylon (i.e., a layer of oxygen barrier material) and causes the surrounding layers to overheat.
As a result of more and more applications requiring high temperature heat seals, a need has arisen for a multi-layer film that includes the heat resistance of Nylon 6,6 without its detrimental effects on the adjacent layers of the structure. The disclosed invention teaches a heat resistant nylon multi-layer film that includes a layer of each of these nylons, and its method of manufacture.